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What Happened To My Dot?

By David Carmichael

April 2, 2018

As corrugated is moving into the digital printing area—sometimes kicking and screaming—the notion of resolution and “clarity” (not the same thing) is a concern. Many ask of the correlation between flexo printing line screen and digital’s dots per inch (dpi). Image resolution is generally stated in one of three ways: dpi for imaging device output (any digital inkjet printer); lines per inch (lpi) for commercial printing with halftone screens (the RDC or FFG in your plant); or pixels per inch (ppi) for actual image resolution (TV, computer, or smartphone screen). By the way, that new iPhone 8 has a resolution of 326 ppi, just for some perspective. There is much written about these measurements. For this article, we are going to focus on corrugated and the differences between lpi and dpi.

Whether digitally printed on corrugated or displayed on your computer screen, a picture is made up of tiny little dots. There are color dots, and there are black dots. Using more dots creates—up to a point—a higher-resolution picture. The more dots in a picture, the larger the size of the graphic computer file. Resolution is measured by the number of dots in a horizontal or vertical inch. Each type of display device (digital printer, monitor) has a maximum number of dots it can process and display, no matter how many dots are in the picture.

Line screen—expressed in lpi—is the measure of how many halftone lines are printed in a linear inch. This important measurement related to the way printers reproduce photographic images also defines the necessary resolution of an image. The lpi is dependent on the output device and the type of paper. To simulate shades of gray using only black ink, a printer prints varying sizes and patterns of halftone spots (spots are made up of many dots of ink). Small halftone dots create the visual illusion of a light gray, while larger halftone dots appear darker, blacker.

Sample print resolutions include:

  • Newsprint—85/110 lpi
  • Web offset—133 lpi
  • Standard sheetfed offset—150 lpi
  • Fine quality—175–200 lpi

Finer dot patterns can be used under ideal conditions. Otherwise they will tend to “plug” (block up) or skip. Flexo printing directly on uncoated corrugated sheets may use very coarse values with very visible dot patterns (30–80 lpi).

For any type of printing, digital in particular, the convergence of variables needs to create an “acceptable” image on the corrugated substrate.

Digital resolution—expressed as dpi—is the usual specification for resolution of output devices, such as desktop printers, film recorders, RIPs (raster image programs), and computer monitors. For computer monitors and film recorders, there is a 1-to-1 ratio between the ppi of the digital image file and the dpi of the output device.

Vector-based images, such as those created in Adobe Illustrator, or most editable fonts, are based upon mathematical curves, not pixels. As such, they are infinitely scalable and will be absolutely sharp at almost any size. But when they are finally output, these vectors are still first converted to the finite elements of the digital printer or flexo plate, just as pixel-based images are. Exceptions include “solid” ink color (unscreened, maximum ink density) in offset printing, such as black type or line work, which have no dot pattern.

For any type of printing, digital in particular, the convergence of variables needs to create an “acceptable” image on the corrugated substrate. The four major variables of digital are jetting, inks, papers, and software (JIPS). Printers with resolutions of 600 dpi or higher may not create a “clear” image due to the effects of the other variables. When considering a digital printer for corrugated, understand the printer is only one piece of the puzzle. The knowledge of the other variables and how they interact with your particular printer is most important. The suppliers of inks, paper, and software—along with the printer’s OEM—should be able to help guide you to the results you expect.

width=700Remember, we mentioned that “clarity” and resolution are not the same. Let’s consider these examples. A 600 dpi laser printer can print up to 600 dots of picture information in an inch. A picture with more dots than the printer can support will waste the extra dots. They increase the digital file size, but don’t improve the printing of the picture; the resolution is too high for that printer. A picture with fewer dots than the printer can support will create a picture without any clarity. For example, a photograph at both 300 dpi and 600 dpi will look the same printed on a 300 dpi printer. The extra dots of information are “thrown out” by the printer, but the 600 dpi picture will have a larger file size. If you print a 72 dpi picture to a 600 dpi printer, it won’t usually look as good as it does on the computer monitor. The printer doesn’t have enough dots of information to create a clear, sharp image. The original file resolution (dpi) determines the printer’s output. Low-resolution input results in low-resolution output, regardless of the printer’s dpi capability.

The clarity issue is even more prominent with printing on corrugated. As noted in the JIPS figure, the paper is a major variable on the digital printer’s capability to print a clear image. On uncoated stock, the small inkjet dot—sometimes as small as 5 picoliters*—will “follow” the paper fibers. What is intended as a drop becomes a sunburst, which can affect clarity, more so than file size (ppi) or printer resolution (dpi).

When considering a digital printer for corrugated, review with each OEM the ink and jetting used. Ensure these characteristics of the printer meet or exceed the needs of your customers using the papers available to you. Have the OEM test print with these available papers. Most OEMs use their own software for machine operation, RIP, and screening processes. However, third-party color management software is available and should be reviewed. These four variables affect the drop size, absorption, and placement impacting the image quality.

So, in reality, nothing happened to your dot. It is still there, but we have more control and a consistent application of it using digital printers. It is an exciting time for the packaging industry.


width=150David Carmichael is technical manager at SUN Automation. He can be reached at 410-472-2900 or david.carmichael@sunautomation.com.